[llvm-dev] LLD PDB Lines zero number issue

Tue Jan 14 04:53:35 PST 2020

OK, maybe I was wrong and the “-use-unknown-locations” option is not the issue root cause.
I'm not familiar with the LLVM codebase and I appreciate any suggestion on how to debug this issue. I'd like to know is it a clang issue or a lld issue? Is there a tool which can dump the obj file debug location info?

Let me simplify the previous test case to reproduce this issue. All what you need is just a C function to invoke an empty assembly function as below.

  1.  An empty assembly function file:
$ cat assembly.nasm
    DEFAULT REL
    SECTION .text
global assembly_fun
assembly_fun:
  ret

  1.  A file of a C function to invoke the empty assembly function but not inline the assembly function:
$ cat main.c
void assembly_fun();
int main()
{
  int Index1;
  for (Index1 = 0; Index1 == 0; assembly_fun(), Index1++){
    assembly_fun();
  }
  assembly_fun();
}

Then link the assembly and c functions together as below:
$ clang -g -m64 -target x86_64-unknown-windows -gcodeview -O1 -c -o main.obj  main.c
$ nasm -f win64 -o assembly.obj assembly.nasm
$ lld-link /OUT:main.dll /Machine:X64 /DLL /ENTRY:main /DEBUG:GHASH /lldmap main.obj assembly.obj

Check to see that there are line zeros in the pdb file:
$ llvm-pdbutil" dump -l main.pdb
                           Lines
============================================================
Mod 0000 | `/home/jshi19/llvm/wrongcode/lld_PDB_issue/main.obj`:
/home/jshi19/llvm/wrongcode/lld_PDB_issue/main.c (MD5: 5F82BB79FE2DA0B0549B784CFDC37D05)
  0001:00000000-0000001A, line/addr entries = 5
     3 00000000 !    0 00000004 !    5 00000009 !    0 0000000E !    9 00000013 !

Thanks
Shi, Steven

From: Zachary Turner <zturner at roblox.com>
Sent: Tuesday, January 14, 2020 2:56 AM
To: Shi, Steven <steven.shi at intel.com>
Cc: Martin Storsjö <martin at martin.st>; David Blaikie <dblaikie at gmail.com>; Rui Ueyama <ruiu at google.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] LLD PDB Lines zero number issue

Could you try putting together a prototype patch and testing it locally to see if it solves your problem?

On Sat, Jan 11, 2020 at 7:11 PM Shi, Steven via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
I checked the LLVM master code and find that the “-use-unknown-locations” option and its related logic is only supported for Dwarf debug info as below. These codes are missing in the Codeview side. It looks the “-use-unknown-locations” option logic is not complex, could we port the code logic into llvm\lib\CodeGen\AsmPrinter\CodeViewDebug.cpp as well? I’ve submitted a BZ for this requirment: https://bugs.llvm.org/show_bug.cgi?id=44522

llvm-project\llvm\lib\CodeGen\AsmPrinter\DwarfDebug.cpp :
static cl::opt<DefaultOnOff> UnknownLocations(
    "use-unknown-locations", cl::Hidden,
    cl::desc("Make an absence of debug location information explicit."),
    cl::values(clEnumVal(Default, "At top of block or after label"),
               clEnumVal(Enable, "In all cases"), clEnumVal(Disable, "Never")),
    cl::init(Default));
... ...
    // If user said Don't Do That, don't do that.
    if (UnknownLocations == Disable)
      return;

This issue blocks the LLVM toolchain debugging capability for UEFI firmware. Please help to add the support. Thank you in advance!

Thanks

Steven Shi

From: Shi, Steven
Sent: Friday, January 10, 2020 9:43 AM
To: Martin Storsjö <martin at martin.st<mailto:martin at martin.st>>; David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>
Cc: Reid Kleckner <rnk at google.com<mailto:rnk at google.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Rui Ueyama <ruiu at google.com<mailto:ruiu at google.com>>
Subject: RE: [llvm-dev] LLD PDB Lines zero number issue

Hi Martin,

I tried your suggestion of adding "-mllvm -use-unknown-locations=Disable" in the clang CC flag to build test case in my previous email, but it looks still not work in my side with llvm 9.0.0.  Could you give me some more advices? Do I need to use the master LLVM instead of the 9.0.0?

$ make -f makefile_clanglto_Oz

"/home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang" -g  -m64 -mcmodel=small -target x86_64-unknown-windows -gcodeview -mllvm -use-unknown-locations=Disable -flto -Oz -c -o main.obj  main.c

"nasm" -f win64 -o assembly.obj assembly.nasm

"/home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/llvm-lib" /OUT:main.lib main.obj

"/home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/llvm-lib" /OUT:assembly.lib assembly.obj

"/home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/lld-link" /OUT:main.dll /Machine:X64 /DLL /ENTRY:main /DEBUG:GHASH /lldmap main.lib assembly.lib

$ /home/jshi19/llvm/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/llvm-pdbutil dump -l main.pdb

                           Lines

============================================================

Mod 0000 | `assembly.obj`:

Mod 0001 | `/home/jshi19/llvm/wrongcode/lld-link5/lto.tmp`:

/home/jshi19/llvm/wrongcode/lld-link5/main.c (MD5: 5F82BB79FE2DA0B0549B784CFDC37D05)

  0001:00000004-0000001E, line/addr entries = 5

     3 00000004 !    0 00000008 !    5 0000000D !    0 00000012 !    9 00000017 !

Thanks

Steven Shi

> -----Original Message-----

> From: Martin Storsjö <martin at martin.st<mailto:martin at martin.st>>

> Sent: Friday, January 10, 2020 5:00 AM

> To: David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>

> Cc: Shi, Steven <steven.shi at intel.com<mailto:steven.shi at intel.com>>; Reid Kleckner <rnk at google.com<mailto:rnk at google.com>>;

> llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>

> Subject: Re: [llvm-dev] LLD PDB Lines zero number issue

>

> On Thu, 9 Jan 2020, David Blaikie via llvm-dev wrote:

>

> > +rnk who works on COFF debugging things.

> >

> >  My understanding is that this has nothing to do with the linker - but is a

> > function of LLVM optimizations creating line zero debug locations. I believe

> > rnk & folks have disabled line zero emission for CodeView debug info in

> some

> > manner - perhaps that needs a flag? or Steven hasn't got a release with

> that

> > change in it?

>

> I think I've run into that with dwarf as well, and there AFAIK it can be

> gotten rid of by adding "-mllvm -use-unknown-locations=Disable".

>

> // Martin
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